It is not easy to send individual photons from an aircraft in a targeted manner, capture them in a ground station, and also detect them. Researchers have now succeeded in doing this: they have even measured various quantum channels between an aircraft and a ground station several times, sent photons to an ion trap, and tested technologies for quantum key distribution. The flight experiment took place as part of the QuNET initiative, which develops technologies for quantum-secure communication. Photons, or particles of light, can be used to generate quantum cryptographic keys that make future communication practically tap-proof. The technologies are also groundbreaking for a future quantum internet that connects quantum computers with each other.

MPL and FAU: Focus on quantum communication

Quantum communication involves the connection of quantum nodes in special networks using methods known as quantum communication protocols. The further development of such protocols and their implementation under realistic conditions has long been a research focus in Prof. Christoph Marquardt’s group at MPL. The group contributed two experiments on quantum communication to the QuNET flight campaign.

Some of the experiments on board include a setup for characterizing the channel properties for quantum key distribution with continuous variables. Here, the quantum information is encoded and measured using methods based on classical optical communication technology.

The second experiment deals with the generation of quantum light, which is coupled from the aircraft to the ion trap of Prof. Gerd Leuchs’ emeritus group on the ground. The quantum light is intended to interact with a single trapped ion there. For this purpose, the aircraft is equipped with a quantum source developed in Erlangen, which, due to its design, is suitable for the optical requirements of the single ion and for use in an aircraft. “Such coupling of quantum light to individual atomic systems is necessary if the nodes of long-range quantum networks are to be connected to each other in the future,” explains Thomas Dirmeier, project manager at MPL.

Scientists from the German Aerospace Center (DLR), the Max Planck Institute for the Science of Light (MPL), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), as well as the Fraunhofer Institutes for Applied Optics and Precision Engineering (IOF) and the Heinrich Hertz Institute (HHI) participated in the experiment. The results have now been presented to the Federal Ministry of Research, Technology, and Space (BMFTR), which funds the QuNET initiative. Quantum key distribution is particularly important for communication between governments and authorities, but also in general for protecting infrastructure and data in everyday life in the future.

“We are working on practical solutions for satellite-based quantum communication, which can be used to transmit quantum states over long distances and generate secure keys. In fiber optics, this is only possible over a few hundred kilometers. Quantum encryption via satellite, on the other hand, enables arbitrarily greater distances on Earth,” says Florian Moll from the DLR Institute of Communications and Navigation, explaining the future technology. To cover long distances, satellites, aircraft, or other mobile platforms are to become part of quantum networks in the future.

The current experiment was flown using a DLR research aircraft from the Flight Experiments facility. The scientists installed an optical communication terminal in the Dornier 228. The aircraft formed a mobile node in a quantum network and established a connection to an optical receiving station on the ground. This ground station is a mobile container with an integrated receiving terminal, known as the QuBUS, provided by Fraunhofer IOF in Jena.

Technically highly complex

Individual photons are difficult to handle: for quantum communication, they must be generated at high quality and be clearly detectable even under strong external interference. For optimal results, the wavelength of the photons must also be adjusted precisely. “We have shown in various experiments that this is possible. The approach we tested can be used not only from aircraft, but also from satellites,” adds Florian Moll. 

The states of the “flying” particles were successfully verified in measurements at the MPL ion trap – which was one of the goals of the experiment. This communication technology could also be used, for example, to connect quantum memories or quantum computers in a future quantum network.

Secure communication for the future

Systems for conducting quantum key distribution (QKD) experiments were connected on the aircraft and at the ground station in Erlangen. These are groundbreaking for satellite-based quantum communication: a system for clock-channel-free quantum key distribution was tested. In addition, the researchers detected photons from an entanglement source on the ground. Channel measurements and component testing for QKD systems with novel and flexibly configurable protocols also provided important insights for further developments in secure communication for the future.

About the QuNET initiative

QuNET (Quantum Network) is a network funded by the Federal Ministry of Research, Technology and Space. (BMFTR) for the research of highly secure communication systems based on quantum communication technologies. QuNET was launched in fall 2019 and was funded for a planned period of seven years. The BMFTR is funding QuNET with 125 million euros. In addition to the DLR Institute of Communications and Navigation, the Fraunhofer Institute for Applied Optics and Precision Engineering IOF, the Fraunhofer Heinrich Hertz Institute (HHI), the Max Planck Institute for the Science of Light (MPL), and the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) are involved.

QuNET aims to lay the foundations for secure and robust IT networks that are already resistant to the cyber attacks of tomorrow. The security of IT communication networks is currently based primarily on mathematical assumptions. These offer protection against future technologies, such as powerful quantum computers.

Scientific Contact

Prof. Christoph Marquardt
Max Planck Institute for the Science of Light
Research Group Leader “Quantum Information Processing”
www.mpl.mpg.de / christoph.marquardt@mpl.mpg.de